The present invention relates to an ink-jet color printing system for forming color images of complex color. More particularly, the invention pertains to the ink-jet color printing system in which a color image faithful to the original color reproduction can be produced by using a non-linear masking technique and by using cyan, magenta and yellow inks in combination in the ink-jet color printing apparatus.
In the conventional ink-jet color printing apparatus for painting color images having complex color such as a color photograph, a color image is painted by means of a dot pattern wherein the injection of colored inks is regulated by driving the ink-jet heads for yellow, magenta and cyan which receive density signals of blue, green and red, respectively. However, in the heretofore known ink-jet color printing apparatus, it is difficult to paint a color image which is faithful to the original color in the original picture having complex color.
It has become clear from investigation of deteriorated images that either insufficient color corrections in the masking procedure or inappropriate combinations of yellow, cyan and magenta inks will cause a deterioration in color reproducibility.
That is to say, a conventional masking procedure by the use of a linear equation having variables which are the color density of three primary colors, namely, blue, green and red, is not desirable for obtaining a fine color correction on account of the fact that there is an extremely high degree of non-linearity between the amount of ink and the optical density.
Inks usually used in ink-jet color printing apparatus contain water-soluble dyes such as direct dyes or acid dyes as described in Japanese Patent Examined Publication Nos. 54-16243, 54-16244, 54-16245 and 54-21765 and Japanese Patent Unexamined Publication Nos. 49-89534, 52-96105, 52-146307, 53-77706 and 54-89811. Between the mixing ratio in which the inks containing water-soluble dyes aforementioned are mixed, and the hue of a color obtained by the mixing, there is a non-linearity in a subtractive mixture, and there is a tendency for the mixed color to appear as coloration unable to be absorbed, even though a polynominal function including non-linear terms is applied to a masking procedure. Therefore, in order to make improvements in color reproducibility, it is necessary to employ inks more suitable in coloration and to carry out a masking procedure having due consideration for the characteristics of the inks.